A variable-speed AC install may lead to issues if the unit isn’t compatible with your ductwork. Incorrectly sized or uninsulated ducts are among the common variable-speed air conditioner problems we see. Duct design has a big impact on system performance. Depending on the current setup, when your HVAC contractor installs a variable-speed system, you may need new ductwork.
Improperly Sized Ducts Must Be Replaced
Variable-speed AC systems are unique in that they adapt to demand, which makes them highly efficient. You won’t achieve your comfort and efficiency goals if air ducts are too small.
A duct system should accommodate 400 cubic feet per minute (CFM) of airflow per ton. Generally, you need a 14-inch wide by 6-inch high rectangular duct to accommodate an air volume of 400 CFM and a 10-inch diameter round duct for the same volume of air. Using this rule, the following size requirements apply:
- A 3-ton AC system requires an air volume of 1,200 CFM. An 8-inch high rectangular duct can support this if it’s 26 inches wide. The required height decreases to 12 inches if the duct is 16 inches wide. An equivalent round duct must be 15 inches in diameter.
- For a 5-ton system, 2,000 CFM is required but can only be supported by a rectangular duct that’s 24 x 10 inches. A round duct must be 18 inches in diameter.
In any case, a duct that is too small won’t pull enough air for your variable-speed air conditioner to work, and the investment in your AC installation won’t pay off.
Is a Ductwork Retrofit an Option?
Replacing or modifying ductwork is a complex decision your HVAC installer must help with. Sharp-angled fittings can be replaced with softer-angled ones to reduce the pressure drop. You may also need fewer linear feet of duct.
If flexible ducts are used, they must be properly supported. Minimizing air resistance can improve efficiency, but if there are issues throughout your ductwork, replacing the entire system may be more cost-effective. Otherwise, you may deal with multiple repairs and long-term inefficiencies.
Duct leaks are a problem too. Mastic can be an effective solution, but consult with your AC installer about other duct sealing methods. Some work from the inside to seal small leaks, while other options, such as spray foam or replacing aging duct segments, maybe more practical. You could also move ducts in unconditioned spaces into the conditioned envelope of your home.
Ductwork Must Accommodate Every Variable-Speed Setting
Unlike single-speed or two-speed air conditioning systems, variable-speed compressors can have hundreds of settings. They usually run at a lower speed. A smaller volume of air is moved if the thermostat setting has been reached. But if your ductwork is too small, the system cannot run efficiently when it must meet a higher demand. That defeats the purpose of purchasing a variable-speed AC unit.
The Importance of Duct Insulation
Variable-speed HVAC systems don’t work well with uninsulated ductwork. Uninsulated or poorly insulated ducts are common issues in older homes. Even a newer home may have uninsulated ductwork in some areas. It’s more commonplace now to insulate it in attics and crawlspaces, but bare metal ducts inside a wall can also lose heat.
That’s a problem with a variable speed system. It runs constantly so ducts stay cold for longer periods. This means a bare metal duct without adequate insulation can start to sweat in the summer. The equivalent of condensation on a drinking glass can cause water leaks, damage to your home, and mold growth.
How Old Is Your Ductwork?
If your ductwork is 10 to 15 years old or older, consider replacing it during your variable-speed AC install. Older ductwork may be incompatible with newer technology. Deteriorating ducts, evident by excessive dust, higher energy bills, and unusual noises, should be replaced. Attempting to repair or insulate them may not be worth the cost when you consider the potential for long-term savings.
Can My Existing Ducts Support a Variable-Speed Air Conditioner?
We often recommend replacing ductwork when upgrading your HVAC system. However, this may not always be the case. You may be able to use the same ductwork if:
- It is sized to support the required airflow.
- It meets the new equipment’s specifications.
- There are no air leaks or other efficiency issues.
- It’s less than a decade old.
- No deterioration or repair issues are present.
- A high-quality insulating material is in place.
Call Monarch Home Services Today!
Throughout the San Joaquin Valley, we provide professional AC replacement and installation. Extensive training and experience enable our technicians to provide a high-quality AC install, whether you need a basic cooling system or want an upgrade, such as a variable-speed air conditioner. Each project includes a load calculation, airflow analysis, and careful attention to ductwork design. To start the planning process and schedule your next HVAC installation, contact us or call (661) 452-8707.